They seem to be suggesting that and it is absurd. The epic Elves of the First Age were much younger than Galadriel is in the Second Age, pretty much at the age of Elrond. We are talking about Elves like Finrod, Fingon, Turgon etc. This suggestion is simply absurd and another canon violation.

You are talking about a writing that's written more as a history book or bible verse, rather then a fully fledged story. These Elves also blew it a number of times. Why shouldn't they be portrayed more three dimensional? And it's not like The Boys where all superheroes are completely terrible people at worst, and tragic terrible people at best. Here, Elves are still the good guys.

I suspect that the birth of their kids might cause Elrond and Galadriel to mature more. Though even there, it’ll probably take a long time… considering Galadriel’s daughter is also the mother of Elrond’s kids.

Have you actually read the Silmarillion? What do The Boys have to do with this? Galadriel's daughter was born at the beggining of the Second Age so that actually is another point bringing down this absurd version her.

And I suppose adaptations have never changed character birthdays? And let me tell you what absurd is. Absurd is thinking that people never change throughout time. If you time traveled to meet your parents when they were younger, you would not recognize them. And again, the book reads like a history book or Bible story. There are countless blank spaces and it’s not really a story. It’s more like the log of a chess game:

I think its less that it takes them longer to mature, and more that they change throughout their lives. Galadriel and Elrond aren’t immature in Rings of Power, but they are different from what they will become.

You seem very confused. This is not a matter of youthful immature years. Galadriel is about 2000 years old when the second Age begins, 3500 when the Rings were forged, 5000 when Ar-Pharazon became king of Numenor (thats how goofed up the timeline of the series is). Tolkien wrote "Elves' bodies developed slower than those of Men, but their minds developed more swiftly. Physical puberty is generally complete by their fiftieth year(by age fifty they reach adult height), but they are not considered full-grown until a hundred years have passed."

*Reminder to avoid crude or vulgar language here.